The present invention relates to a scroll pump, which is often referred to as a scroll compressor.
A prior art scroll compressor, or pump, 100 is shown in FIG. 7. The pump 100 comprises a pump housing 102 and a drive shaft 104 having an eccentric shaft portion 106. The shaft 104 is driven by a motor 108 and the eccentric shaft portion is connected to an orbiting scroll 110 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 112 for pumping fluid along a fluid flow path between a pump inlet 114 and pump outlet 116 of the compressor.
The fixed scroll 112 comprises a scroll wall 118 which extends perpendicularly to a generally circular base plate 120. The orbiting scroll 110 comprises a scroll wall 124 which extends perpendicularly to a generally circular base plate 126. The orbiting scroll wall 124 co-operates, or meshes, with the fixed scroll wall 118 during orbiting movement of the orbiting scroll. Relative orbital movement of the scrolls causes a volume of gas to be trapped between the scrolls and pumped from the inlet to the outlet.
A scroll may be used as a vacuum pump for example for evacuating a process chamber in which semiconductor products are processed. The scroll may be arranged in series with a high vacuum pump such as a turbo molecular pump or may be connected directly to a process chamber. When initial evacuation is commenced the inlet and the exhaust of the scroll pump are at atmosphere. This initial phase is often referred to as roughing and a scroll pump used in this way is referred to as a roughing pump. During roughing, gas is compressed by the scroll pump, but since the inlet is initially at atmosphere, the pump may generate over-compression in the pump. Over-compression in this context means that a pressure is generated in the pump which is above atmosphere. Over-compression is undesirable because it increases the load on the pump and therefore increases the power requirement of the pump motor.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.